Electronic and pneumatic controllers are commonly used for controlling physical states of process variables, such as pressure, temperature, level and other variables for process and industrial operations. The simplest type of control system uses an on/off controller, commonly known as bang-bang control. For example, see U.S. Pat. No. 3,778,205, issued Dec. 11, 1973 to Lane, et al, entitled Modified On-Off Control.
For processes having one or more process variables which respond quickly to changes in the final control element, better results can be obtained by a controller with proportional action, the output of which changes in proportion to a change in the state of the process variable. The gain setting on a proportional controller provides a means for adjusting the ratio of controller output change to change in the state of the process variable, so that the control action can be tuned to the process control loop. Low gain makes a controller react slowly to cause the state of the process variable to return to set point when the state of the process variable moves away from the set point, while a high gain setting causes the controller to act more like an on/off controller discussed above. A potential deficiency of the proportional controller is that it generates steady state offsets of the state of the process variable with respect to the set point for load changes. To compensate for this potential deficiency, integral action may be added to proportional action to overcome offset. The integral portion of the controller compensates for average error over a period of time. If the error exists, it will continue to add to the action of the proportional portion of the controller output to correct the error or offset (this action is also called reset action). Therefore, an adjustment is needed to change the frequency with which the integral portion of the controller resets (repeats per minute) the proportional portion. Integral action is usually used in conjunction with proportional action, and the two together are referred to as a "PI controller".
While a PI controller can help eliminate an offset, it can cause larger correction overshoots than proportional control. Another type of control action, called derivative action, may be necessary to overcome or minimize this potential deficiency. Derivative action provides a controller output proportional to the rate of change of the error signal between the set point and the state of the process variable. The adjustment for derivative action ("rate"), if set too low, will cause slow response to load changes and, if set too high, will cause control system instability. The rate is therefore usually adjusted for each control system so that the system responds correctly to changes in load. The resulting instrument is known as a "PID controller", or a three mode controller.
The tuning of this controller requires skill, and it takes considerable time and effort to achieve a stable system.
For general reference to controllers with regard to final control elements, see U.S. Pat. No. 2,231,568, issued Feb. 11, 1941 to H.H. Gorrie, entitled Control System; U.S. Pat. No. 2,917,066, issued Dec. 15, 1959 to G. Bergson, entitled Fluid Flow Control System; U.S. Pat. No. 3,196,900, issued Jul. 27, 1965 to A.R. Catheron, et al, entitled Electronic Control Apparatus; U.S. Pat. No. 3,307,824, issued Mar. 7, 1967 to G. Weisheit, entitled Control System for Flowing Media; U.S. Pat. No. 4,146,051, issued Mar. 27, 1979 to Brian E. Sparkes, entitled Fluid Flow Control System; U.S. Pat. No. 4,431,020, issued Feb. 14, 1984 to Kowalski, entitled Flow-Control System Having a Wide Range of Flow-Rate Control.
When a controller is used in conjunction with a diaphragm control valve, another device called a positioner is normally used between controller and the valve for precise control of a process variable.
For various types of control valves, see U.S. Pat. No. Re. 32,644, reissued Apr. 12, 1988 by Brundage, et al, entitled Solenoid Control Flow Valve; U.S. Pat. No. 2,398,452, issued Apr. 16, 1946 to Shaw, entitled 3-Way Solenoid Valve; U.S. Pat. No. 2,616,449, issued Nov. 4, 1952 to Maha, entitled Pilot Operated Solenoid Control Valve; U.S. Pat. No. 3,135,493, issued Jun. 2, 1964 to Gizeski, entitled Electro-Pneumatic Valve Operator; U.S. Pat. No. 3,211,415, issued Oct. 12, 1965 to Rudelick, entitled pilot Control Valve Actuating Mechanism; U.S. Pat. No. 4,058,287, issued Nov. 15, 1977 to Fromfield, entitled Pilot-Operated Valve Having Constant Closing Rate; U.S. Pat. No. 4,553,732, issued Nov. 19, 1985 to Brundage, et al, entitled Solenoid Control Flow Valve; U.S. Pat. No. 4,605,197, issued Aug. 12, 1986 to Casey, et al, entitled Proportional and Latching Pressure Control Device; U.S. Pat. No. 4,699,351, issued Oct. 13, 1987 to Wells, entitled Pressure Responsive Pilot Actuated Modulating Valve; U.S. Pat. No. 3,926,405 , issued Dec. 16, 1975 to Arnold, entitled Solenoid Operated Proportional Valve; U.S. Pat. No. 4,014,509, issued Mar. 29, 1977 to Yoshino, et al, entitled Proportional Electro Magnetic-Type Direction and Throttle-Controlling Valve; U.S. Pat. No. 4,049,232, issued Sep. 20, 1977 to Byers, entitled Pressure Compensating Fluid Control Valve; U.S. Pat. No. 4,193,421, issued Mar. 18, 1982 to Sakakibara, et al, entitled Electromagnetically Operated Valve Unit; U.S. Pat. No. 4,411,406, issued Oct. 25, 1983 to Inada, et al, entitled Electro Magnetic Flow Control Valve Assembly; U.S. Pat. No. 4,585,206, issued Apr. 29, 1986 to Itoh, entitled Proportional Flow Control Valve; U.S. Pat. No. 4,605,197, issued Aug. 12, 1986 to Casey, et al, entitled Proportional and Latching Pressure Control Valve; U.S. Pat. No. 4,729,397, issued Mar. 8, 1988 to Bruss, entitled Electro Magnetic Control Valve for a Pressure Fluid and Associated Method.
Although it has been known in the art to control pneumatic operated valves with pneumatic controllers and to use interfaces with pneumatic valves, including solenoid valves, to drive a pneumatic valve by an electronic controller, and it has also been known to use electronic transmitters and transducers to supply the state of process variables to electronic controllers, the conversion from electronic to pneumatic signals is expensive. There is, accordingly, a need for an electronic controller sensing electronic signals representative of the state of process variables and directly these signals as output modifications from a proportional action and directly interfacing to pneumatic control valves, such as diaphragm control valves, without an intermediary positioner.